reprocess-daemon$Mtcp_local$1M$1~-error$4000000?Temporary lookup failure
In this rewrite rule, localhost is the host name associated with the local channel. The value of the DOMAIN_FAILURE option shown here is the default value so it does not need to appear in option.dat under normal circumstances.
The ordering here is especially tricky. The MTA checks $V after the address is rebuilt but before the route is added. This lets the MTA to change the route in the event of a temporary lookup failure. Pending channel match checks are applied any time the insertion point changes, so the @ after the second $H invokes the check. If the check succeeds, the remainder of the template applies and rewrite processing concludes. If the check fails, the rewrite fails and rewriting continues with the next applicable rewrite rule. If the check cannot be performed due to a temporary failure, template processing continues with the value specified by the DOMAIN_FAILURE MTA option. The value of this template first sets the routing host to reprocess-daemon. Then the template checks to see whether or not the MTA is dealing with a reprocessing channel of some sort or tcp_local. If the MTA is dealing with such a channel, the rule continues, making the routing host illegal and specifying a temporary failure as the outcome. If the MTA is not dealing with such a channel, the rule is truncated and successfully terminated, thereby rewriting the address to the reprocess channel.